Acyl coenzyme A: cholesterol acyltransferase (ACAT) is an intracellular enzyme that uses cholesterol and fatty acyl-coenzyme A (CoA) to form cholesterol esters. Accumulation of cholesterol esters as cytoplasmic lipid droplets within cells of human aortic tissue is a characteristic feature of early lesions of atherosclerotic plaque. In intestines of vertebrate animals, the extent of absorption of dietary cholesterol can be shown to be significantly reduced by inhibiting intestinal ACAT activity. In livers of vertebrate animals, formation of lipoproteins require proper supply of cholesterol esters produced through the ACAT catalyzed reaction.
ACAT is a membrane-bound enzyme located in the endoplasmic reticulum of various tissues of animal and human cells. The enzyme has been localized to the rough endoplasmic reticulum in rat liver. It is highly regulated in many cell types and tissues, and it is believed to play an important role in cholesterol metabolism in various cells and tissues such as the small intestinal mucosa, hepatocytes, macrophages, and the steroid hormone-producing tissues (O'Brien, P. M. and Sliskovic, D. R. (1992) in Current Opinion in Therapeutic Patents; Cadigan, K. M., et al. (1988) J. Biol. Chem. 263:274-282; Cadigan, K. M., et al. (1989) J. Cell Biol. 108:2201-2210).
Although ACAT has been studied intensively, much remains to be learned about its molecular structure. The active site of the enzyme has been localized to the cytoplasmic surface of the microsomal vesicles in the rat liver, using a combination of detergent and protease treatments, but whether the enzyme spans the entire membrane has not yet been determined. Lichtenstein, A. H. and Brecher, P. (1980) J. Biol. Chem. 255:9098-9104. Recent chemical modification studies indicate that essential histidyl and sulfhydryl residues may reside at or near the active site of the enzyme. Studies of ACAT activities of rabbit tissues suggest the existence of different ACAT subtypes since various tissues have differing sensitivities to histidyl-modifying reagents. Kinnunen, P. M. et al. (1988) Biochemistry 27:7344-7350.
ACAT activity has been studied from ACAT solubilized and reconstituted from various cultured cells, including rat and pig liver cells. Although these procedures have allowed enzyme activity to be measured in a defined lipid environment, little progress has been made as yet in purifying the solubilized preparations. To date, no laboratory had succeeded in purifying ACAT to homogeneity with retention of biological activity.